This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objective: To determine the effects of leukocyte co-culture on embryo development in vitro. At the time of implantation and throughout pregnancy, the primate uterus contains numerous leukocytes, consisting primarily of natural killer cells and macrophages. The cytokine balance at the maternal fetal interface and the expression of non-classical MHC class I molecules (HLA-G in humans and Mamu-AG in the rhesus monkey) on the trophoblast cells of the growing embryo are thought to be important immune modulators that allow the semi-allogenic embryo to implant and grow. However, study of the maternal fetal interface in human implantation is not feasible. We have established a co-culture system with rhesus monkey embryos to model implantation. Rhesus blastocysts stage embryos, derived by in vitro fertilization, were co-cultured with peripheral blood NK cells and/or monocytes obtained from the oocyte donor. Embryo growth was monitored and culture media were collected to determine cytokine and chorionic gonadotropin secretion. Trophoblast outgrowths from the embryo were noticeable after 6 days of co-culture. Embryos co-cultured with monocytes did not show signs of growth within the two weeks of co-culture, however embryos co-cultured with both monocytes and NK cells grew at similar rates to controls. Based on these results we conclude that the monocyte/NK cell balance and/or interaction is important and that using a rhesus model may lead to increased knowledge of the role of leukocytes during pregnancy.